1. Field of the Invention
The present invention relates to an imaging system utilizing a solid-state imaging device and, more particularly, to a method of constructing an optical low pass filter passing only a lower spatial frequency using a phase grating and the structure of the grating. The present invention provides a method for removing an image with a higher spatial frequency using a two-dimensionally arranged phase grating. The invention proposes a method of constructing the optical low pass filter employing the two-dimensionally arranged phase grating for the purpose of removing an image with a higher spatial frequency in an imaging system utilizing a semiconductor solid-state imaging device such as CCD image sensor or CMOS image sensor, and provides structures of the grating for realizing the proposed optical low pass filter.
2. Description of the Related Art
In a charge coupled device (CCD) image sensor currently widely used as an image sensor or CMOS image sensor that has been used since 90s, the image sensors configured of light receiving elements are two-dimensionally arranged to convert input images into electrical signals. FIG. 1 shows an ideal sampling in case where the repetitive period of the light receiving element is X in direction x and Y in the direction y in the two-dimensional image sensor. If an image having the spatial frequency spectrum of FIG. 2A is imaged using the two-dimensional sensor having the spatial sampling characteristic of FIG. 1, the sampled image has the spatial frequency spectrum of FIG. 2B in which the original image""s spatial frequency spectrum is repeated. In FIG. 2B, the frequency spectrum of the sampled image has a repetitive period corresponding to the reciprocal of the sampling interval, that is, 1/X in the x-direction and 1/Y in the y-direction. Accordingly, to restore the image inputted to the two-dimensional image sensor to the original state, it requires an optical low pass filter which passes the spectrum corresponding to one period starting from the starting point but cuts off a spatial frequency higher than this.
FIG. 3 shows the configuration of a conventional camcorder or digital camera system. A motion picture or still picture 1 to be imaged is focused by an optical lens arrangement 2 and then passes through an optical low pass filter 3 to enter a light receiving element constructed on the surface of an image sensor 4. The optical lens arrangement 2 consists of appropriate optical lenses such as concave lens and convex lens in order to focus the input image 1 on the imaging device. The optical lens arrangement 2 of FIG. 3 usually includes an UV filter or IR filter for respectively blocking ultraviolet rays or infrared rays contained in the input image. The UV or IR filter is generally constructed in a manner that an appropriate material is coated on a lens or a transparent glass plate.
As described above, to restore the image inputted to the solid-state imaging device to the original state, it is ideal that the optical low pass filter 3 of FIG. 3 has a cutoff frequency that is one-half the sampling spatial frequency. Here, the sampling spatial frequency corresponds to the reciprocal of the repetitive period of the light receiving element of the solid-state imaging device. That is, in the two-dimensional light receiving element arrangement of FIG. 1, fs=1/d and fc=fs/2=1/2d, where d is X in the x-direction and Y in the y-direction. Here, fs represents the sampling frequency and fc. represents the cutoff frequency of an ideal optical low pass filter.
FIG. 4 shows the spatial frequency transfer characteristic of the optical lens arrangement. The frequency band defined by a dotted line in FIG. 4 is the frequency transfer function of an ideal optical low pass filter. The maximum transfer frequency of the lens, fm, is 2(NA/xcex). Here, NA represents the numerical aperture of the lens and xcex represents the wavelength of incident light. Though the lens functions as a kind of optical low pass filter, its maximum cutoff frequency, fm, is usually considerably: higher than the ideal cutoff frequency, fc, of the low pass filter as shown in FIG. 4. The frequency transfer characteristic of the lens can approximate to the straight line of FIG. 4 to be mathematically modeled, and the difference between the approximate value indicated by the straight line and the actual transfer characteristic becomes smaller as fm becomes larger than fc.
FIGS. 5A, 5B and 5C illustrate conventional optical low pass filters utilizing a double refraction plate, which are currently widely used as an optical low pass filter. Referring to FIG. 5A, an input beam incident on one surface of the double refraction plate is split into two beams, having a distance, dn, therebetween, while it passes through the double refraction plate. The relation among the thickness and refraction index of the double refraction plate and the distance, dn, satisfies the following equation:       d    n    =            t      ⁡              (                              n            e            2                    -                      n            o            2                          )                    2      ⁢              n        e            ⁢              n        o            
where t is the thickness of the double refraction plate, ne is the extra-ordinary refraction index and no is the ordinary refraction index. As shown in FIG. 5B, the conventional optical low pass filter utilizing the double refraction plate is constructed in such a manner that an x-directional double refraction plate and y-directional double refraction plate lie in piles to enable beam splitting in the x-direction and y-direction. An IR removal filter is generally inserted between the two double refraction plates.
In the operation of the conventional optical low pass filter utilizing the double refraction plate, the input beam, vertically incident on the surface of the filter, is split into two beams at the x-directional double refraction plate, and each of these two beams is further split into two beams at the y-direction double refraction plate. Thus, one input beam is split into four beams, arriving at the light receiving element of the solid-state imaging device. That is, the optical low pass filter using the double refraction plate functions as a 4-beam splitter as shown in FIG. 5C. By splitting one input beam into four beams, an image having a higher spatial frequency is converted into a lower spatial frequency before sampling of the solid-state imaging device.
The general optical transfer characteristic function of 2-plate type double refraction plate is equals to the magnitude of the absolute value of the cosine function with the period of 1/dn when it is Fourier-transformed. That is, the transfer function has a value proportional to abs(cos(2xcfx80xc3x97fxc3x97dn)) where f is spatial frequency and dn is the distance between the beams split by the double refraction plate.
FIG. 6 shows the optical transfer function of the double refraction plate filter when dn=d. The optical transfer function of an image which passes through the optical lens to reach the double refraction plate filter is obtained by multiplying the transfer function of the lens shown in FIG. 4 by the transfer function of the double refraction plate. The optical transfer function corresponds to the solid line of FIG. 6.
In case where the double refraction plate is applied to the conventional imaging system utilizing the solid-state imaging device, larger loss generates in the transfer function in a spatial frequency band lower than the cutoff frequency than in the ideal optical low pass filter. This loss deteriorates the resolution of the image sensor. Furthermore, there exists a periodic transfer function in a spatial frequency band higher than the cutoff frequency so that a higher frequency component cannot be removed. This generates aliasing to thereby bring about afterglow. In other words, the optical low pass filter utilizing the double refraction plate has larger loss in a lower band and larger surplus portion in a higher band, resulting in deterioration of resolution and poor effect on the removal of afterglow.
There have been proposed optical phase grating low pass filters having various structures as shown in FIGS. 7A to 7E for the purpose of improving the performance of the conventional optical low pass filter using the double refraction plate. FIG. 7A shows the vertical grating filter described in U.S. Pat. No. 4,083,627, FIG. 7B represents the circular grating filter proposed in U.S. Pat. No. 4,083,627, and FIG. 7C illustrates the lozenge-shaped grating filter disclosed in U.S. Pat. No. 4,009,939. FIG. 7D shows the parallel repetitive grating filter proposed in U.S. Pat. No. 4,795,236 and No. 4,178,611, and FIG. 4E illustrates the optical phase grating low pass filter constructed in a manner that gratings having a refraction index different from that of a grating substrate whose both surfaces are used are repeatedly arranged in parallel, disclosed in U.S. Pat. No. 4,795,236.
However, most of the aforementioned optical phase grating low pass filters are not being actually utilized since they could not be manufactured. This is because the optical phase grating low pass filters of FIGS. 7A to 7E have the structures that two gratings having different phase shifts from each other are two-dimensionally arranged. Thus, according to computer simulation and Fourier transform carried out by this inventor, their performances are not remarkably improved compared to the conventional low pass filter using the double refraction plate. That is, the conventional optical phase grating low pass filter has the disadvantage that the transfer characteristic of the spatial frequency spectrum is not much improved compared to the conventional filter utilizing the double refraction plate because the optical phase grating low pass filter has the two gratings having different phases. This is why the optical phase grating low pass filter proposed for the purpose of improving the conventional optical low pass filter using the double refraction plate cannot be practically utilized.
It is, therefore, an object of the present invention to provide an optical phase grating low pass filter which increases the optical transfer function at a frequency band lower than the ideal cutoff frequency corresponding to one-half the sampling spatial frequency of a solid-state imaging device but suppresses the transfer function at a band higher than the cutoff frequency.
To accomplish the object of the present invention, there is provided an optical low pass filter which suppresses a spatial frequency component higher than a specific frequency and passes a component lower than the specific frequency in an imaging system sensing input images, the optical low pass filter comprising: a 0-phase shift grating for generating the phase shift of 0; a xcfx86-phase shift grating for generating the phase shift of xcfx86, arranged at the right and bottom of the 0-phase shift grating; and a 2xcfx86-phase shift grating for generating the phase shift of 2xcfx86, arranged at the diagonal side of the 0-phase shift grating.
To accomplish the object of the present invention, there is provided an optical low pass filter which suppresses a spatial frequency component higher than a specific frequency and passes a component lower than the specific frequency in an imaging system sensing input images, the optical low pass filter comprising: a plurality of basic arrangement patterns periodically arranged, wherein each of the basic arrangement pattern consists of: a 0-phase shift grating for generating the phase shift of 0; a xcfx86-phase shift grating for generating the phase shift of xcfx86, arranged at the right and bottom of the 0-phase shift grating; and a 2xcfx86-phase shift grating for generating the phase shift of 2xcfx86, arranged at the diagonal side of the 0-phase shift grating.
To accomplish the object of the present invention, there is also provided an optical low pass filter which suppresses a spatial frequency component higher than a specific frequency and passes a component lower than the specific frequency in an imaging system sensing input images, the optical low pass filter comprising: a plurality of basic arrangement patterns periodically arranged, wherein each of the basic arrangement pattern consists of: a xcfx86-phase shift grating for generating the phase shift of xcfx86, having a predetermined thickness and formed on a transparent grating substrate; a 2xcfx86-phase shift grating for generating the phase shift of 24, having a thickness twice of the xcfx86-phase shift grating and formed on the same grating substrate; and a portion for generating the phase shift of 0, having no grating.
To accomplish the object of the present invention, there is also provided An optical low pass filter which suppresses a spatial frequency component higher than a specific frequency and passes a component lower than the specific frequency in an imaging system sensing input images, the optical low pass filter comprising: a first grating for generating the phase shift of xcfx86, having a predetermined thickness and periodically arranged on one transparent grating substrate in the horizontal direction; and a second grating for generating the phase shift of xcfx86, having a predetermined thickness and periodically arranged on the other transparent grating substrate in the vertical direction, wherein the surfaces of the first and second gratings are attached to each other facing each other, to thereby construct a structure in which a xcfx86-phase shift grating for generating the xcfx86-phase shift, a 2xcfx86-phase shift grating for generating the 2xcfx86-phase shift, and a 0-phase shift grating are periodically arranged between the two transparent grating substrates.
The optical low pass filter has a filter for blocking UV rays or IR rays which is formed on one of the top and bottom faces of the attached grating substrate structure. The optical low pass filter may have a filter for blocking UV rays or IR rays which is formed on each of the top and bottom faces of the attached grating substrate structure. The optical low pass filter also may have a filter for blocking UV rays which is formed on one of the top and bottom faces of the attached grating substrate structure, and a filter for blocking IR rays which is formed on the other face.
To accomplish the object of the present invention, there is provided An optical low pass filter which suppresses a spatial frequency component higher than a specific frequency and passes a component lower than the specific frequency in an imaging system sensing input images, the optical low pass filter comprising: a first grating for generating the phase shift of xcfx86, having a predetermined thickness and periodically arranged on one transparent grating substrate in the horizontal direction; and a second grating for generating the phase shift of xcfx86, having a predetermined thickness and periodically arranged on the other transparent grating substrate in the vertical direction, wherein the surfaces of the two grating substrates on which the gratings are not formed are attached to each other facing each other, to thereby construct a structure in which a xcfx86-phase shift grating for generating the xcfx86-phase shift, a 2xcfx86-phase shift grating for generating the 2xcfx86-phase shift and a 0-phase shift grating are periodically arranged.
The optical low pass filter has a filter for blocking UV rays or IR rays which is inserted between the attached surfaces of the two grating substrates.
To accomplish the object of the present invention, there is also provided An optical low pass filter which suppresses a spatial frequency component higher than a specific frequency and passes a component lower than the specific frequency in an imaging system sensing input images, the optical low pass filter comprising: a first grating for generating the phase shift of xcfx86, having a predetermined thickness and periodically arranged on one surface of a transparent grating substrate in the horizontal direction; and a second grating for generating the phase shift of xcfx86, having a predetermined thickness and periodically arranged on the other surface of the transparent grating substrate in the vertical direction, wherein the first and second gratings and the grating substrate are formed of materials having the same refraction index, to thereby construct a structure in which a xcfx86-phase shift grating for generating the xcfx86-phase shift, a 2xcfx86-phase shift grating for generating the 2xcfx86-phase shift, and a 0-phase shift grating are periodically arranged.